AIM Station Datums

[ZLP955]

The AIM diagrams sometimes include measurements from reference stations. An example would be the placement of emblems on body panels, and includes both longitudinal and transverse dimension references. Unless I've missed it, I couldn't find in the AIM where the datum (starting) point for these stations is. Can anyone clarify?

[Mark]

For the horizontal ones it's the firewall, negative numbers are forward of the firewall, positive ones are behind the firewall.  Vertical ones are off some part of the equipment the car was carried on, because the 0 refernce is about 5" below the bottom of the rocker panel.  Theres also some referenced off the car centerline, like the U73 rear mounted antenna mounting hole.

[ZLP955]

Thanks for the reply Mark, appreciate the info.

[JohnZ]

The reference dimensions are part of the GM 3-dimensional body grid system, and none of the zero datums are identifiable physical points on the body; the datums are only meaningful when the body is set up on a surface plate to the design master datums, which the plant did two or three times a day for match-metal checks to document body-in-white dimensional accuracy and to track any trends away from nominal that might require a tooling adjustment in the Body Shop.

[ZLP955]

John, great info (as always). Were any of those reference points physically transferred onto the car during assembly? Otherwise, how were emblems and other items positioned to the AIM dimensions by the line workers?
(I do understand that the AIM is an engineering document, and not all details were for the worker's use/benefit).

[JohnZ]

John, great info (as always). Were any of those reference points physically transferred onto the car during assembly? Otherwise, how were emblems and other items positioned to the AIM dimensions by the line workers?
(I do understand that the AIM is an engineering document, and not all details were for the worker's use/benefit).

Locating fixtures, drill fixtures, etc. were designed/developed on a match-metal body set up on the surface plate to the design 3-D dimensions with trammeling height gages. It was quite rare to have any locating dimensions called out in the A.I.M. as "on surface" - 99% were expressed in 3-D in X-Y-Z datum format.